1. Field of the Invention
This invention generally relates to apparatus for optically recording and/or reproducing information on and/or from an optical disk, and, in particular, to an optical pick-up device for use in such apparatus. More specifically, the present invention relates to an optical pick-up device having a structure which is not adversely affected by fluctuations in the ambient temperature. In addition, the present invention also relates to a semiconductor laser device suitable for use as a light source in such an optical information recording and/or reproducing apparatus.
2. Description of the Prior Art
An optical information recording and/or reproducing apparatus is well known in the art and it generally includes an optical recording medium, typically an optical disk, and an optical pick-up. A relative motion is provided between the optical recording medium and the optical pick-up, whereby information is recorded onto the optical recording medium or recorded information is optically read from the optical recording medium by the optical pick-up. For this purpose, the optical pick-up is typically provided with a light source, such as a semiconductor laser device, from which a laser beam is emitted to be projected onto the recording medium. Typically, the recording medium is in the shape of a disk which is driven to rotate in a predetermined direction and a recording track, either spiral or circular, is previously defined on a recording surface of the disk. Thus, it is required that the light beam emitted from the optical pick-up device be not only properly focused onto the recording medium, but also properly track the recording track of the recording medium.
However, a support member for supporting the semiconductor laser and the optical system, including a coupling lens for collimating the laser beam emitted from the laser, varies its length depending on the ambient temperature, so that the distance between the laser and the coupling lens changes depending on the ambient temperature. On the other hand, the wavelength of the laser beam emitted from the laser also changes depending on the ambient temperature. Accordingly, the focusing condition of the optical pick-up device, in particular that of the coupling lens can be significantly affected by the ambient condition. In this case, there is produced an artificial error in a focus error signal. Several proposals have been made to compensate the variation in distance between the laser and the coupling lens due to thermal expansion of their support member when the ambient temperature changes as disclosed in Japanese Patent Laid-open Publications Nos. 59-15206, 60-107743 and 60-205839. However, these techniques are not satisfactory because none of them takes into consideration the dependency of the wavelength of an emitted laser beam on the ambient temperature.